A wind turbine generator is typically made of these parts: a tower; a nacelle sitting at the top of the tower including generators, gearboxes or any means to convert wind energy to electrical energy; a hub with a pitch bearing for each blade; and a plurality, e.g. two or three, of wind turbine blades each fitted to a respective pitch bearing of the hub. A tidal generator correspondingly includes a hub and blades fitted thereto.
The “root” of the blade is the end attached to the hub via the pitch bearing and is typically cylindrically annular, and shaped and dimensioned to mate with the bearing. Blades are commonly built using composite materials, in particular fibre reinforced composite materials comprising fibres within a resin matrix, and so the composite material root must be attached to the pitch bearing which is composed of metal. The attachment is usually achieved by bolting the root to the bearing so that the blade may be mounted and dismounted.
It is necessary for the root attachment system to be able reliably, in use, to carry both tensile and compression forces which are alternately imposed thereon as the wind turbine blade rotates during operation of the wind turbine.
Typically the blade is joined to the pitch bearing by a circular array of bolts extending circumferentially around the annular root, for example about 50-100 bolts for a large blade, typically up to 55 m long. There are three common methods in use for attaching the bolts to the composite structure of the blade:
T-bolts: as shown in FIG. 1, for each bolt 300 extending from the pitch bearing (not shown), a cylindrical metal insert 302 is fitted into a radially extending hole 304 extending through the thickness of the annular blade root 306. The inserts 302 are typically 2-3 times the diameter of the bolt 300. The insert 302 is drilled and tapped with a helical thread to accept the helically threaded bolt 300, which is inserted through a longitudinally extending hole 308 in the free end 310 of the root 306. The root 306 must be thick to have sufficient bearing strength to prevent pull-out of the inserts 302. The thickness means the root 306 is heavy and is a problem for manufacture due to exotherm of the resin in the composite material. This system is currently used by most blade manufacturers using resin infusion to produce the fibre reinforced composite material root.
Bonded bushes: as shown in FIG. 2, for each bolt extending from the pitch bearing (not shown), a longitudinal hole 402, larger than the diameter of the bolt, is drilled in the free end of the composite material root 404. A cylindrical bush 406 is adhered into the hole 402. The bush 406 is internally threaded to accept the bolt. The bond strength of the bushes 406 to the composite root 404 is critical, requiring careful manufacture. The fibre reinforced composite material laminate needs to be thick to be strong enough even after the holes 402 are drilled, causing problems with exotherm of the resin as described above.
Bushes laminated-in: as shown in FIG. 3, for each bolt extending from the pitch bearing (not shown), a longitudinally extending bush 502 is disposed within the root 504, extending inwardly from the free end 506, the bushes 502 being included within the laminated composite material during the lamination process, rather than being adhered in afterwards as for the bonded bushes system described above. This allows the root laminate to be much thinner, as it naturally follows the shape of the bushes 502 without needing unnecessary composite material between them. Hence this solution is lighter and less prone to exotherm than (a) and (b). Foam 510 is provided between the adjacent laminate portions 512 annularly surrounding the bushes 502. However it is complicated to laminate, hence labour costs can be high.